tikzpicture "There is no ... in font nullfont", but no extraneous text

If you take my tikz answer and add 0.5pt between the two \draw commands, you get no error, and the output looks the same but the log warns:

Missing character: There is no 0 in font nullfont!
Missing character: There is no . in font nullfont!
Missing character: There is no 5 in font nullfont!
Missing character: There is no p in font nullfont!
Missing character: There is no t in font nullfont!

If you want to see where the error is, you can make \nullfont have the characters add

\font\nullfont=cmr10

Then again you get no error but this time no warning in the log, but the output is very messed up and you can see the offending characters.

Don't use that redefinition of \nullfont in production code but as a temporary redefinition while debugging, it can be useful.

After testing your example, these warnings seem to be the result of the \multiply commands in the code of \pgf@arrowset. The expression after by in a \multiply command must be an integer. If it expands to a decimal number the . and the digits following are seen as text. The following changes should be made: change

\multiply\pgflinewidth by \pgflinewidth@arrow\relax

to

\pgflinewidth = \pgflinewidth@arrow \pgflinewidth \relax

and

\multiply\pgflinewidth by \noexpand\pgflinewidth@arrow\relax

to

\pgflinewidth = \noexpand\pgflinewidth@arrow \pgflinewidth \relax

These use the fact that a dimension (\pdflinewidth in this case) can be preceded by a factor (stored in the macro \pgflinewidth@arrow in this case) to effect a multiplication. I don't know much about pgf, so I can't say that this will be free of other problems.